Provided by: qemu-utils_4.2-3ubuntu6.30_amd64 
NAME
qemu-img - QEMU disk image utility
SYNOPSIS
qemu-img [standard options] command [command options]
DESCRIPTION
qemu-img allows you to create, convert and modify images offline. It can handle all image
formats supported by QEMU.
Warning: Never use qemu-img to modify images in use by a running virtual machine or any
other process; this may destroy the image. Also, be aware that querying an image that is
being modified by another process may encounter inconsistent state.
OPTIONS
Standard options:
-h, --help
Display this help and exit
-V, --version
Display version information and exit
-T, --trace [[enable=]pattern][,events=file][,file=file]
Specify tracing options.
[enable=]pattern
Immediately enable events matching pattern (either event name or a globbing
pattern). This option is only available if QEMU has been compiled with the
simple, log or ftrace tracing backend. To specify multiple events or patterns,
specify the -trace option multiple times.
Use "-trace help" to print a list of names of trace points.
events=file
Immediately enable events listed in file. The file must contain one event name
(as listed in the trace-events-all file) per line; globbing patterns are accepted
too. This option is only available if QEMU has been compiled with the simple, log
or ftrace tracing backend.
file=file
Log output traces to file. This option is only available if QEMU has been
compiled with the simple tracing backend.
The following commands are supported:
amend [--object objectdef] [--image-opts] [-p] [-q] [-f fmt] [-t cache] -o options
filename
bench [-c count] [-d depth] [-f fmt] [--flush-interval=flush_interval] [-n] [--no-drain]
[-o offset] [--pattern=pattern] [-q] [-s buffer_size] [-S step_size] [-t cache] [-w] [-U]
filename
check [--object objectdef] [--image-opts] [-q] [-f fmt] [--output=ofmt] [-r [leaks | all]]
[-T src_cache] [-U] filename
commit [--object objectdef] [--image-opts] [-q] [-f fmt] [-t cache] [-b base] [-d] [-p]
filename
compare [--object objectdef] [--image-opts] [-f fmt] [-F fmt] [-T src_cache] [-p] [-q]
[-s] [-U] filename1 filename2
convert [--object objectdef] [--image-opts] [--target-image-opts] [-U] [-C] [-c] [-p] [-q]
[-n] [-f fmt] [-t cache] [-T src_cache] [-O output_fmt] [-B backing_file] [-o options] [-l
snapshot_param] [-S sparse_size] [-m num_coroutines] [-W] [--salvage] filename [filename2
[...]] output_filename
create [--object objectdef] [-q] [-f fmt] [-b backing_file] [-F backing_fmt] [-u] [-o
options] filename [size]
dd [--image-opts] [-U] [-f fmt] [-O output_fmt] [bs=block_size] [count=blocks]
[skip=blocks] if=input of=output
info [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [--backing-chain] [-U]
filename
map [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [-U] filename
measure [--output=ofmt] [-O output_fmt] [-o options] [--size N | [--object objectdef]
[--image-opts] [-f fmt] [-l snapshot_param] filename]
snapshot [--object objectdef] [--image-opts] [-U] [-q] [-l | -a snapshot | -c snapshot |
-d snapshot] filename
rebase [--object objectdef] [--image-opts] [-U] [-q] [-f fmt] [-t cache] [-T src_cache]
[-p] [-u] -b backing_file [-F backing_fmt] filename
resize [--object objectdef] [--image-opts] [-f fmt] [--preallocation=prealloc] [-q]
[--shrink] filename [+ | -]size
Command parameters:
filename
is a disk image filename
fmt is the disk image format. It is guessed automatically in most cases. See below for a
description of the supported disk formats.
size
is the disk image size in bytes. Optional suffixes "k" or "K" (kilobyte, 1024) "M"
(megabyte, 1024k) and "G" (gigabyte, 1024M) and T (terabyte, 1024G) are supported.
"b" is ignored.
output_filename
is the destination disk image filename
output_fmt
is the destination format
options
is a comma separated list of format specific options in a name=value format. Use "-o
?" for an overview of the options supported by the used format or see the format
descriptions below for details.
snapshot_param
is param used for internal snapshot, format is 'snapshot.id=[ID],snapshot.name=[NAME]'
or '[ID_OR_NAME]'
--object objectdef
is a QEMU user creatable object definition. See the qemu(1) manual page for a
description of the object properties. The most common object type is a "secret", which
is used to supply passwords and/or encryption keys.
--image-opts
Indicates that the source filename parameter is to be interpreted as a full option
string, not a plain filename. This parameter is mutually exclusive with the -f
parameter.
--target-image-opts
Indicates that the output_filename parameter(s) are to be interpreted as a full option
string, not a plain filename. This parameter is mutually exclusive with the -O
parameters. It is currently required to also use the -n parameter to skip image
creation. This restriction may be relaxed in a future release.
--force-share (-U)
If specified, "qemu-img" will open the image in shared mode, allowing other QEMU
processes to open it in write mode. For example, this can be used to get the image
information (with 'info' subcommand) when the image is used by a running guest. Note
that this could produce inconsistent results because of concurrent metadata changes,
etc. This option is only allowed when opening images in read-only mode.
--backing-chain
will enumerate information about backing files in a disk image chain. Refer below for
further description.
-c indicates that target image must be compressed (qcow format only)
-h with or without a command shows help and lists the supported formats
-p display progress bar (compare, convert and rebase commands only). If the -p option is
not used for a command that supports it, the progress is reported when the process
receives a "SIGUSR1" or "SIGINFO" signal.
-q Quiet mode - do not print any output (except errors). There's no progress bar in case
both -q and -p options are used.
-S size
indicates the consecutive number of bytes that must contain only zeros for qemu-img to
create a sparse image during conversion. This value is rounded down to the nearest 512
bytes. You may use the common size suffixes like "k" for kilobytes.
-t cache
specifies the cache mode that should be used with the (destination) file. See the
documentation of the emulator's "-drive cache=..." option for allowed values.
-T src_cache
specifies the cache mode that should be used with the source file(s). See the
documentation of the emulator's "-drive cache=..." option for allowed values.
Parameters to snapshot subcommand:
snapshot
is the name of the snapshot to create, apply or delete
-a applies a snapshot (revert disk to saved state)
-c creates a snapshot
-d deletes a snapshot
-l lists all snapshots in the given image
Parameters to compare subcommand:
-f First image format
-F Second image format
-s Strict mode - fail on different image size or sector allocation
Parameters to convert subcommand:
-n Skip the creation of the target volume
-m Number of parallel coroutines for the convert process
-W Allow out-of-order writes to the destination. This option improves performance, but is
only recommended for preallocated devices like host devices or other raw block
devices.
-C Try to use copy offloading to move data from source image to target. This may improve
performance if the data is remote, such as with NFS or iSCSI backends, but will not
automatically sparsify zero sectors, and may result in a fully allocated target image
depending on the host support for getting allocation information.
--salvage
Try to ignore I/O errors when reading. Unless in quiet mode ("-q"), errors will still
be printed. Areas that cannot be read from the source will be treated as containing
only zeroes.
Parameters to dd subcommand:
bs=block_size
defines the block size
count=blocks
sets the number of input blocks to copy
if=input
sets the input file
of=output
sets the output file
skip=blocks
sets the number of input blocks to skip
Command description:
amend [--object objectdef] [--image-opts] [-p] [-q] [-f fmt] [-t cache] -o options
filename
Amends the image format specific options for the image file filename. Not all file
formats support this operation.
bench [-c count] [-d depth] [-f fmt] [--flush-interval=flush_interval] [-n] [--no-drain]
[-o offset] [--pattern=pattern] [-q] [-s buffer_size] [-S step_size] [-t cache] [-w] [-U]
filename
Run a simple sequential I/O benchmark on the specified image. If "-w" is specified, a
write test is performed, otherwise a read test is performed.
A total number of count I/O requests is performed, each buffer_size bytes in size, and
with depth requests in parallel. The first request starts at the position given by
offset, each following request increases the current position by step_size. If
step_size is not given, buffer_size is used for its value.
If flush_interval is specified for a write test, the request queue is drained and a
flush is issued before new writes are made whenever the number of remaining requests
is a multiple of flush_interval. If additionally "--no-drain" is specified, a flush is
issued without draining the request queue first.
If "-n" is specified, the native AIO backend is used if possible. On Linux, this
option only works if "-t none" or "-t directsync" is specified as well.
For write tests, by default a buffer filled with zeros is written. This can be
overridden with a pattern byte specified by pattern.
check [--object objectdef] [--image-opts] [-q] [-f fmt] [--output=ofmt] [-r [leaks | all]]
[-T src_cache] [-U] filename
Perform a consistency check on the disk image filename. The command can output in the
format ofmt which is either "human" or "json". The JSON output is an object of QAPI
type "ImageCheck".
If "-r" is specified, qemu-img tries to repair any inconsistencies found during the
check. "-r leaks" repairs only cluster leaks, whereas "-r all" fixes all kinds of
errors, with a higher risk of choosing the wrong fix or hiding corruption that has
already occurred.
Only the formats "qcow2", "qed" and "vdi" support consistency checks.
In case the image does not have any inconsistencies, check exits with 0. Other exit
codes indicate the kind of inconsistency found or if another error occurred. The
following table summarizes all exit codes of the check subcommand:
0 Check completed, the image is (now) consistent
1 Check not completed because of internal errors
2 Check completed, image is corrupted
3 Check completed, image has leaked clusters, but is not corrupted
63 Checks are not supported by the image format
If "-r" is specified, exit codes representing the image state refer to the state after
(the attempt at) repairing it. That is, a successful "-r all" will yield the exit code
0, independently of the image state before.
commit [--object objectdef] [--image-opts] [-q] [-f fmt] [-t cache] [-b base] [-d] [-p]
filename
Commit the changes recorded in filename in its base image or backing file. If the
backing file is smaller than the snapshot, then the backing file will be resized to be
the same size as the snapshot. If the snapshot is smaller than the backing file, the
backing file will not be truncated. If you want the backing file to match the size of
the smaller snapshot, you can safely truncate it yourself once the commit operation
successfully completes.
The image filename is emptied after the operation has succeeded. If you do not need
filename afterwards and intend to drop it, you may skip emptying filename by
specifying the "-d" flag.
If the backing chain of the given image file filename has more than one layer, the
backing file into which the changes will be committed may be specified as base (which
has to be part of filename's backing chain). If base is not specified, the immediate
backing file of the top image (which is filename) will be used. Note that after a
commit operation all images between base and the top image will be invalid and may
return garbage data when read. For this reason, "-b" implies "-d" (so that the top
image stays valid).
compare [--object objectdef] [--image-opts] [-f fmt] [-F fmt] [-T src_cache] [-p] [-q]
[-s] [-U] filename1 filename2
Check if two images have the same content. You can compare images with different
format or settings.
The format is probed unless you specify it by -f (used for filename1) and/or -F (used
for filename2) option.
By default, images with different size are considered identical if the larger image
contains only unallocated and/or zeroed sectors in the area after the end of the other
image. In addition, if any sector is not allocated in one image and contains only zero
bytes in the second one, it is evaluated as equal. You can use Strict mode by
specifying the -s option. When compare runs in Strict mode, it fails in case image
size differs or a sector is allocated in one image and is not allocated in the second
one.
By default, compare prints out a result message. This message displays information
that both images are same or the position of the first different byte. In addition,
result message can report different image size in case Strict mode is used.
Compare exits with 0 in case the images are equal and with 1 in case the images
differ. Other exit codes mean an error occurred during execution and standard error
output should contain an error message. The following table sumarizes all exit codes
of the compare subcommand:
0 Images are identical
1 Images differ
2 Error on opening an image
3 Error on checking a sector allocation
4 Error on reading data
convert [--object objectdef] [--image-opts] [--target-image-opts] [-U] [-C] [-c] [-p] [-q]
[-n] [-f fmt] [-t cache] [-T src_cache] [-O output_fmt] [-B backing_file] [-o options] [-l
snapshot_param] [-S sparse_size] [-m num_coroutines] [-W] filename [filename2 [...]]
output_filename
Convert the disk image filename or a snapshot snapshot_param to disk image
output_filename using format output_fmt. It can be optionally compressed ("-c" option)
or use any format specific options like encryption ("-o" option).
Only the formats "qcow" and "qcow2" support compression. The compression is read-only.
It means that if a compressed sector is rewritten, then it is rewritten as
uncompressed data.
Image conversion is also useful to get smaller image when using a growable format such
as "qcow": the empty sectors are detected and suppressed from the destination image.
sparse_size indicates the consecutive number of bytes (defaults to 4k) that must
contain only zeros for qemu-img to create a sparse image during conversion. If
sparse_size is 0, the source will not be scanned for unallocated or zero sectors, and
the destination image will always be fully allocated.
You can use the backing_file option to force the output image to be created as a copy
on write image of the specified base image; the backing_file should have the same
content as the input's base image, however the path, image format, etc may differ.
If a relative path name is given, the backing file is looked up relative to the
directory containing output_filename.
If the "-n" option is specified, the target volume creation will be skipped. This is
useful for formats such as "rbd" if the target volume has already been created with
site specific options that cannot be supplied through qemu-img.
Out of order writes can be enabled with "-W" to improve performance. This is only
recommended for preallocated devices like host devices or other raw block devices. Out
of order write does not work in combination with creating compressed images.
num_coroutines specifies how many coroutines work in parallel during the convert
process (defaults to 8).
create [--object objectdef] [-q] [-f fmt] [-b backing_file] [-F backing_fmt] [-u] [-o
options] filename [size]
Create the new disk image filename of size size and format fmt. Depending on the file
format, you can add one or more options that enable additional features of this
format.
If the option backing_file is specified, then the image will record only the
differences from backing_file. No size needs to be specified in this case.
backing_file will never be modified unless you use the "commit" monitor command (or
qemu-img commit).
If a relative path name is given, the backing file is looked up relative to the
directory containing filename.
Note that a given backing file will be opened to check that it is valid. Use the "-u"
option to enable unsafe backing file mode, which means that the image will be created
even if the associated backing file cannot be opened. A matching backing file must be
created or additional options be used to make the backing file specification valid
when you want to use an image created this way.
The size can also be specified using the size option with "-o", it doesn't need to be
specified separately in this case.
dd [--image-opts] [-U] [-f fmt] [-O output_fmt] [bs=block_size] [count=blocks]
[skip=blocks] if=input of=output
Dd copies from input file to output file converting it from fmt format to output_fmt
format.
The data is by default read and written using blocks of 512 bytes but can be modified
by specifying block_size. If count=blocks is specified dd will stop reading input
after reading blocks input blocks.
The size syntax is similar to dd(1)'s size syntax.
info [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [--backing-chain] [-U]
filename
Give information about the disk image filename. Use it in particular to know the size
reserved on disk which can be different from the displayed size. If VM snapshots are
stored in the disk image, they are displayed too.
If a disk image has a backing file chain, information about each disk image in the
chain can be recursively enumerated by using the option "--backing-chain".
For instance, if you have an image chain like:
base.qcow2 <- snap1.qcow2 <- snap2.qcow2
To enumerate information about each disk image in the above chain, starting from top
to base, do:
qemu-img info --backing-chain snap2.qcow2
The command can output in the format ofmt which is either "human" or "json". The JSON
output is an object of QAPI type "ImageInfo"; with "--backing-chain", it is an array
of "ImageInfo" objects.
"--output=human" reports the following information (for every image in the chain):
image
The image file name
file format
The image format
virtual size
The size of the guest disk
disk size
How much space the image file occupies on the host file system (may be shown as 0
if this information is unavailable, e.g. because there is no file system)
cluster_size
Cluster size of the image format, if applicable
encrypted
Whether the image is encrypted (only present if so)
cleanly shut down
This is shown as "no" if the image is dirty and will have to be auto-repaired the
next time it is opened in qemu.
backing file
The backing file name, if present
backing file format
The format of the backing file, if the image enforces it
Snapshot list
A list of all internal snapshots
Format specific information
Further information whose structure depends on the image format. This section is
a textual representation of the respective "ImageInfoSpecific*" QAPI object (e.g.
"ImageInfoSpecificQCow2" for qcow2 images).
map [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [-U] filename
Dump the metadata of image filename and its backing file chain. In particular, this
commands dumps the allocation state of every sector of filename, together with the
topmost file that allocates it in the backing file chain.
Two option formats are possible. The default format ("human") only dumps known-
nonzero areas of the file. Known-zero parts of the file are omitted altogether, and
likewise for parts that are not allocated throughout the chain. qemu-img output will
identify a file from where the data can be read, and the offset in the file. Each
line will include four fields, the first three of which are hexadecimal numbers. For
example the first line of:
Offset Length Mapped to File
0 0x20000 0x50000 /tmp/overlay.qcow2
0x100000 0x10000 0x95380000 /tmp/backing.qcow2
means that 0x20000 (131072) bytes starting at offset 0 in the image are available in
/tmp/overlay.qcow2 (opened in "raw" format) starting at offset 0x50000 (327680). Data
that is compressed, encrypted, or otherwise not available in raw format will cause an
error if "human" format is in use. Note that file names can include newlines, thus it
is not safe to parse this output format in scripts.
The alternative format "json" will return an array of dictionaries in JSON format. It
will include similar information in the "start", "length", "offset" fields; it will
also include other more specific information:
- whether the sectors contain actual data or not (boolean field "data"; if false,
the sectors are either unallocated or stored as optimized all-zero clusters);
- whether the data is known to read as zero (boolean field "zero");
- in order to make the output shorter, the target file is expressed as a "depth";
for example, a depth of 2 refers to the backing file of the backing file of
filename.
In JSON format, the "offset" field is optional; it is absent in cases where "human"
format would omit the entry or exit with an error. If "data" is false and the
"offset" field is present, the corresponding sectors in the file are not yet in use,
but they are preallocated.
For more information, consult include/block/block.h in QEMU's source code.
measure [--output=ofmt] [-O output_fmt] [-o options] [--size N | [--object objectdef]
[--image-opts] [-f fmt] [-l snapshot_param] filename]
Calculate the file size required for a new image. This information can be used to
size logical volumes or SAN LUNs appropriately for the image that will be placed in
them. The values reported are guaranteed to be large enough to fit the image. The
command can output in the format ofmt which is either "human" or "json". The JSON
output is an object of QAPI type "BlockMeasureInfo".
If the size N is given then act as if creating a new empty image file using qemu-img
create. If filename is given then act as if converting an existing image file using
qemu-img convert. The format of the new file is given by output_fmt while the format
of an existing file is given by fmt.
A snapshot in an existing image can be specified using snapshot_param.
The following fields are reported:
required size: 524288
fully allocated size: 1074069504
The "required size" is the file size of the new image. It may be smaller than the
virtual disk size if the image format supports compact representation.
The "fully allocated size" is the file size of the new image once data has been
written to all sectors. This is the maximum size that the image file can occupy with
the exception of internal snapshots, dirty bitmaps, vmstate data, and other advanced
image format features.
snapshot [--object objectdef] [--image-opts] [-U] [-q] [-l | -a snapshot | -c snapshot |
-d snapshot] filename
List, apply, create or delete snapshots in image filename.
rebase [--object objectdef] [--image-opts] [-U] [-q] [-f fmt] [-t cache] [-T src_cache]
[-p] [-u] -b backing_file [-F backing_fmt] filename
Changes the backing file of an image. Only the formats "qcow2" and "qed" support
changing the backing file.
The backing file is changed to backing_file and (if the image format of filename
supports this) the backing file format is changed to backing_fmt. If backing_file is
specified as "" (the empty string), then the image is rebased onto no backing file
(i.e. it will exist independently of any backing file).
If a relative path name is given, the backing file is looked up relative to the
directory containing filename.
cache specifies the cache mode to be used for filename, whereas src_cache specifies
the cache mode for reading backing files.
There are two different modes in which "rebase" can operate:
Safe mode
This is the default mode and performs a real rebase operation. The new backing
file may differ from the old one and qemu-img rebase will take care of keeping the
guest-visible content of filename unchanged.
In order to achieve this, any clusters that differ between backing_file and the
old backing file of filename are merged into filename before actually changing the
backing file.
Note that the safe mode is an expensive operation, comparable to converting an
image. It only works if the old backing file still exists.
Unsafe mode
qemu-img uses the unsafe mode if "-u" is specified. In this mode, only the backing
file name and format of filename is changed without any checks on the file
contents. The user must take care of specifying the correct new backing file, or
the guest-visible content of the image will be corrupted.
This mode is useful for renaming or moving the backing file to somewhere else. It
can be used without an accessible old backing file, i.e. you can use it to fix an
image whose backing file has already been moved/renamed.
You can use "rebase" to perform a "diff" operation on two disk images. This can be
useful when you have copied or cloned a guest, and you want to get back to a thin
image on top of a template or base image.
Say that "base.img" has been cloned as "modified.img" by copying it, and that the
"modified.img" guest has run so there are now some changes compared to "base.img". To
construct a thin image called "diff.qcow2" that contains just the differences, do:
qemu-img create -f qcow2 -b modified.img diff.qcow2
qemu-img rebase -b base.img diff.qcow2
At this point, "modified.img" can be discarded, since "base.img + diff.qcow2" contains
the same information.
resize [--object objectdef] [--image-opts] [-f fmt] [--preallocation=prealloc] [-q]
[--shrink] filename [+ | -]size
Change the disk image as if it had been created with size.
Before using this command to shrink a disk image, you MUST use file system and
partitioning tools inside the VM to reduce allocated file systems and partition sizes
accordingly. Failure to do so will result in data loss!
When shrinking images, the "--shrink" option must be given. This informs qemu-img that
the user acknowledges all loss of data beyond the truncated image's end.
After using this command to grow a disk image, you must use file system and
partitioning tools inside the VM to actually begin using the new space on the device.
When growing an image, the "--preallocation" option may be used to specify how the
additional image area should be allocated on the host. See the format description in
the "NOTES" section which values are allowed. Using this option may result in
slightly more data being allocated than necessary.
NOTES
Supported image file formats:
raw Raw disk image format (default). This format has the advantage of being simple and
easily exportable to all other emulators. If your file system supports holes (for
example in ext2 or ext3 on Linux or NTFS on Windows), then only the written sectors
will reserve space. Use "qemu-img info" to know the real size used by the image or "ls
-ls" on Unix/Linux.
Supported options:
"preallocation"
Preallocation mode (allowed values: "off", "falloc", "full"). "falloc" mode
preallocates space for image by calling posix_fallocate(). "full" mode
preallocates space for image by writing data to underlying storage. This data may
or may not be zero, depending on the storage location.
qcow2
QEMU image format, the most versatile format. Use it to have smaller images (useful if
your filesystem does not supports holes, for example on Windows), optional AES
encryption, zlib based compression and support of multiple VM snapshots.
Supported options:
"compat"
Determines the qcow2 version to use. "compat=0.10" uses the traditional image
format that can be read by any QEMU since 0.10. "compat=1.1" enables image format
extensions that only QEMU 1.1 and newer understand (this is the default). Amongst
others, this includes zero clusters, which allow efficient copy-on-read for sparse
images.
"backing_file"
File name of a base image (see create subcommand)
"backing_fmt"
Image format of the base image
"encryption"
If this option is set to "on", the image is encrypted with 128-bit AES-CBC.
The use of encryption in qcow and qcow2 images is considered to be flawed by
modern cryptography standards, suffering from a number of design problems:
- The AES-CBC cipher is used with predictable initialization vectors based on
the sector number. This makes it vulnerable to chosen plaintext attacks which
can reveal the existence of encrypted data.
- The user passphrase is directly used as the encryption key. A poorly chosen or
short passphrase will compromise the security of the encryption.
- In the event of the passphrase being compromised there is no way to change the
passphrase to protect data in any qcow images. The files must be cloned, using
a different encryption passphrase in the new file. The original file must then
be securely erased using a program like shred, though even this is ineffective
with many modern storage technologies.
- Initialization vectors used to encrypt sectors are based on the guest virtual
sector number, instead of the host physical sector. When a disk image has
multiple internal snapshots this means that data in multiple physical sectors
is encrypted with the same initialization vector. With the CBC mode, this
opens the possibility of watermarking attacks if the attack can collect
multiple sectors encrypted with the same IV and some predictable data. Having
multiple qcow2 images with the same passphrase also exposes this weakness
since the passphrase is directly used as the key.
Use of qcow / qcow2 encryption is thus strongly discouraged. Users are recommended
to use an alternative encryption technology such as the Linux dm-crypt / LUKS
system.
"cluster_size"
Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster sizes
can improve the image file size whereas larger cluster sizes generally provide
better performance.
"preallocation"
Preallocation mode (allowed values: "off", "metadata", "falloc", "full"). An image
with preallocated metadata is initially larger but can improve performance when
the image needs to grow. "falloc" and "full" preallocations are like the same
options of "raw" format, but sets up metadata also.
"lazy_refcounts"
If this option is set to "on", reference count updates are postponed with the goal
of avoiding metadata I/O and improving performance. This is particularly
interesting with cache=writethrough which doesn't batch metadata updates. The
tradeoff is that after a host crash, the reference count tables must be rebuilt,
i.e. on the next open an (automatic) "qemu-img check -r all" is required, which
may take some time.
This option can only be enabled if "compat=1.1" is specified.
"nocow"
If this option is set to "on", it will turn off COW of the file. It's only valid
on btrfs, no effect on other file systems.
Btrfs has low performance when hosting a VM image file, even more when the guest
on the VM also using btrfs as file system. Turning off COW is a way to mitigate
this bad performance. Generally there are two ways to turn off COW on btrfs: a)
Disable it by mounting with nodatacow, then all newly created files will be NOCOW.
b) For an empty file, add the NOCOW file attribute. That's what this option does.
Note: this option is only valid to new or empty files. If there is an existing
file which is COW and has data blocks already, it couldn't be changed to NOCOW by
setting "nocow=on". One can issue "lsattr filename" to check if the NOCOW flag is
set or not (Capital 'C' is NOCOW flag).
Other
QEMU also supports various other image file formats for compatibility with older QEMU
versions or other hypervisors, including VMDK, VDI, VHD (vpc), VHDX, qcow1 and QED.
For a full list of supported formats see "qemu-img --help". For a more detailed
description of these formats, see the QEMU Emulation User Documentation.
The main purpose of the block drivers for these formats is image conversion. For
running VMs, it is recommended to convert the disk images to either raw or qcow2 in
order to achieve good performance.
SEE ALSO
The HTML documentation of QEMU for more precise information and Linux user mode emulator
invocation.
AUTHOR
Fabrice Bellard
2024-10-20 QEMU-IMG.1(1)